1. Field of the Invention
This invention generally relates to electro-optic converters and in particular to integrated optical waveguides and photodetectors/modulators for converting between optical and electrical signals.
2. Description of the Related Art
Ultra fast photodetectors and optical modulators are required for high frequency fiber optic links. However, ultra fast photodetectors/modulators tend to saturate at very low optical power because of the need for small detector sizes and high optical power density. Velocity-Matched-Distributed-Photodiodes (VMDP) have been developed to overcome these problems. In a VMDP high power handling capability is achieved by combining a string of photodetectors. The photocurrent signals from each detector add in phase when the velocities of the optical wave that illuminates the photodetectors and the velocity of the electrical wave on the transmission line coupled to the photodetectors are equal.
U.S. Pat. No. 5,572,014 entitled xe2x80x9cHighly Efficient, Ultra fast Optical-to-Electrical Converter and Method for Operating Samexe2x80x9d and Issued on Nov. 5, 1996 discloses one such VMDP device. In the ""014 device a plurality of identical PIN diodes are arrayed along a GaAs waveguide, which also serves as the intrinsic layer of the PIN diodes. The diodes are electrically coupled with a transmission line, and the velocity of the electrical signal propagating along the transmission line is matched with the group velocity of the optical wave in the waveguide by an appropriate choice of the capacitance of each of the photodiodes. The device offers the promise of a VMDP with a high degree of integration. There are, however, several drawbacks to this and other prior art devices. First, their efficiency is limited to a maximum of 50%. Second, they are difficult to fabricate. The first limitation results from the requirement that the traveling wave structure set up in the transmission line by the photodetectors requires two ports of termination, i.e., the actual load and a dummy load which matches the actual load in resistance. Second, in an effort to couple the waveguide with the photodetectors, discontinuities are introduced into the waveguide. In the case of the above mentioned ""014 patent, the waveguide tapers to a very thin layer or quantum well in the intrinsic layer of each photodiode. This tapering of the waveguide introduces scattering in the light propagating along the waveguide, which further reduces the efficiency of the device. Additionally, the variations in the core thickness are not easily achieved and may result in optical discontinuities or contamination.
What is needed is an VMDP with improved efficiency, and manufacturability.
The present invention provides a VMDP converter for converting between an optical signal and an electrical signal. The converter has twice the theoretical efficiency of any prior art device. The converter has improved manufacturability when compared with prior art devices since the wave guide core is of substantially uniform cross-sectional thickness along the optical path. Additionally, current contribution from individual photodetectors may be individually tuned to more evenly distribute the opto-electric conversion process.
In an embodiment of the invention the converter for converting between an optical signal and an electrical signal includes a passive optical waveguide and a plurality of photodiodes. The passive optical waveguide conveys the optical signal. The photodiodes optically couple in series with the passive optical waveguide and electrically couple in parallel with one another to convey the electrical signal there between. The photodiodes exhibit impedance mismatches with respect to one another. These mismatches generate reflections of the electrical signal which contribute to a cancellation of reverse traveling portions of the electrical signal.
In an alternate embodiment of the invention the converter also includes a passive optical waveguide and photodiodes. The passive optical waveguide includes a band gap tuned to exhibit substantial transparency to a characteristic wavelength of the optical signal at a field strength less than a first electrical field strength and to exhibit substantial absorption of the optical signal at the field strength greater than the first field strength. The photodiodes are displaced from one another along the passive optical waveguide. The photodiodes are electrically coupled in parallel with one another to convey the electrical signal there between. The plurality of photodiodes exhibit bias voltages sufficient to generate within said passive optical waveguide localized electric fields with a field strength greater than the first electrical field strength to optically couple the photodiodes with the passive optical waveguide.
In an alternate embodiment, a method for converting between an optical signal in a passive optical waveguide and an electrical signal is disclosed. The method includes:
adjusting a bandgap of the passive optical waveguide to exhibit substantial transparency to a characteristic wavelength of the optical signal at an electrical field strength less than a first electrical field strength and to exhibit substantial absorption of the optical signal at an electrical field strength greater than the first electrical field strength; and
generating within said passive optical waveguide localized regions with electric fields exhibiting an electrical field strength greater than the first electrical field strength; to couple the optical signal with the electrical signal.